1. Field of the Invention
The present invention relates to a radio-frequency amplifier which is operated at high efficiency, especially a radio-frequency amplifier which is operated in a high frequency band and for which the linear characteristic is maintained up to a high power area, and a radio communication system employing such a radio-frequency amplifier.
2. Description of the Related Art
In a radio communication system, such as a mobile communication system employing the wide-band CDMA (Code Division Multiple Access) system or the TDMA (Time Division Multiple Access) system, communication must be performed in an assigned frequency band (channel). In this radio communication system, a desired modulation for a transmission signal is acquired by using a transmission circuit, the obtained signal, which occupies a narrow band, is amplified by a radio-frequency (RF) amplifier, and the resultant signal is transmitted via an antenna. Therefore, for a radio communication system, a radio-frequency amplifier is desired for which the linear characteristic can be maintained up into a high power area, and a broad dynamic range can be obtained.
FIGS. 1A to 1C are diagrams illustrating a communication system employing a conventional radio-frequency amplifier. As is shown in FIG. 1A, a radio-frequency signal RF1 is generated by a transmission circuit in a wide-band CDMA system, and is amplified by a power amplifier PA. The spread of the spectrum of an output signal RF2 is restricted or removed by a band-pass filter 2, and a radio-frequency signal RF3 occupying a narrow band is transmitted via an antenna 3. Normally, the amplifier PA has a satisfactory linear characteristic in a low input power area, while in a high input power area, the output power is saturated and the characteristic is non-linear. Therefore, if the dynamic range is to be employed in order to efficiently operate the amplifier PA, the spectrum of the output signal RF2 produced by the amplifier PA is spread so that it exceeds the limits of a desired band (channel) and interferes with the transmission of communication signals in adjacent channels.
Specifically, as is shown in FIG. 1B, the transmission signal RF1 generated by the transmission circuit 1 is fitted within a narrow band (channel), while the output signal RF2 of the amplifier PA, due to the non-linear characteristic of the amplifier PA, is spread and extends outward into adjacent frequency bands. Thus, conventionally, a band-pass filter 2 is inserted, at the output side of the amplifier PA, which permits the passage only of the part of a signal that fits within a desired band. With this band-pass filter 2, portions of a spectrum that extend outward into adjacent frequency bands is suppressed, and as is shown in FIG. 1C, the power exerted by the signal RF3 in adjacent bands is reduced.
However, the suppression characteristic of a band-pass filter 2, which is normally made of metal, is not ideal, and does not have a sharp skirt characteristic with a desired band as the center, so that the removal of the spread of the spectrum of the signal RF3 is not satisfactory. Therefore, in order to adequately eliminate the spread, multiple stage band-pass filters are provided. This arrangement, however, is not preferable because there is an increased power loss in the desired band. Of course, if the power input to the amplifier PA is backed off (lowered) from the level at which the amplifier becomes saturated, the spread of the spectrum can be removed; but in this case, the efficiency of the radio-frequency amplifier is deteriorated.
Therefore, as the band-pass filter 2, the use has been proposed of a superconducting band-pass filter wherein a superconducting material having a sharp skirt characteristic and a low power loss, even if multiple filters are connected together, is employed for a strip line. As one such proposal, disclosed in Japanese Unexamined Patent Publication No. Hei 9-261082 is the employment of a superconducting band-pass filter that provides a sharp cutoff characteristic.
However, an efficient operation and a linear characteristic can not be attained merely by inserting a superconducting band-pass filter into a radio-frequency amplifier. That is, in order for a power amplifier PA to operate efficiently, the input power that is used must originate in the high power range, without being backed off from the saturation level, and as large a dynamic range as possible must be employed. Therefore, the power of the radio-frequency signal RF2 which is transmitted to the superconducting band-pass filter 2 must be increased.
As a result, a high power and high frequency signal is provided along the transmission path of the superconducting band-pass filter 2 which, because its main component is a superconducting material, is normally stored in a freezer at a temperature considerably lower than the critical temperature of the superconducting material. Then, when the high power radio-frequency signal RF arrives at the filter, the resistance along the transmission path composed of the superconducting material, is increased and heat is generated, especially within an area of either end of the transmission path where the electromagnetic waves are concentrated. Subsequently, the temperature along the transmission path rises until it exceeds the critical temperature of the superconducting material, and the high conductivity quality of the superconducting material is lost. Furthermore, if the superconducting material which is used for the transmission path is made of a ceramic, not only will the high conductivity be lost, but eventually the material will acquire an insulating characteristic. Therefore, the insertion loss for the suppercounductor band-pass filter is increased for a high power and high frequency signal.
As is described above, according to the conventional method, the power resistance of superconducting film can not prevent the superconducting characteristic of a superconducting band-pass filter from being adversely affected by the power of a signal RF2 output by the power amplifier PA, and can not satisfactorily suppress the spread of the spectrum of the signal RF3 which is to be transmitted.
In addition, if lightning should strike an antenna and a very high power charge should thereby be supplied to the superconducting filter, superconducting film employed therein would be destroyed. And since after the film is destroyed the filter can no longer function as a superconducting filter, it must be replaced. In such a case, because the conductive characteristic of superconducting filters is constantly variable, the radio communication system for which the new filter was provided would have to be adjusted, and radio communication would be interrupted.